Visit Great Wall on Return Trip

Hi, my brother and I are flying from LAX to MEL with a large layover in Beijing on the flight to Melbourne and also on the return. Just want to make sure that we qualify for the 72 hour Visa and also that we can visit the Great Wall on the return layover. Not sure if the 72 hour period would start when we first arrive in Beijing, or if the period starts when we apply for the entry permit on the return trip.

Thanks!